Electric resistance heating elements



Nov. 17, 1959 P. BORGHULT ETAL ELECTRIC RESISTANCE HEATING ELEMENTS 5Sheets-Sheet 1 Filed July 5;. use

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8: gOHN HELGE HAGLUND INVENTORS PER BORGHULT ATTO R NEYS Nov. 17, 1959P. BORGHULT EI'AL 2,913,691?

ELECTRIC RESISTANCE HEATING ELEMENTS Filed July 5, 1956 5 Sheets-Sheet 2INVENTORS PER BORGHULT 8x J'OHN HELGE HAGLUND ATTOBN E YS P. BORGHULTETAL' 2,913,695 ELECTRIC RESISTANCE HEATING ELEMENTS Nov. 17, 1959 1 5Sheets-Sheet 3 Filed July 5, 1956 Fig. 4

1 INVENTORS PER BORGHULT JOHN HELGE HAGLUND ATTORNEYS P. BORGHULT ETALELECTRIC RESISTANCE HEATING ELEMENTS Nov. 17,1959 I 5 Sheets-SheetAFiled July 5, 1956 INVENTORS PER BORGHULT ATTORNEYS I Nov. 17,1959 P.BORGHULT ET L 2,913,695

ELECTRIC RESISTANCE HEATING ELEMENTS Filed July 5, 1956 5 Sheets-Sheet 5{50 7/ VZ LQ w X f 2 J v j q 5'45 uth ;"'1 i INVENTORS PER BORGHULT 8JOHN HELGE HAGLUND rrpmvzvs United States Patent ELECTRIC RESISTANCEI-HEATlNG ELEMENTS Per Borghult, Enskede, and John Helge Haglund,Hallstahammar, Sweden, assignors to Aktiebolaget Kanthal, Hallstahammar,Sweden Application July 5, 1956, Serial No. 596,012 Claims priority,application Sweden July 11, 1955 13 Claims. (Cl. 338-317) The presentinvention relates to mounting arrangements for electric resistanceheating elements for electric resistance furnaces operating attemperatures above 1000' C., preferably however, up to at least 1400 C.

It has already been proposed to use, for operating temperature of thisorder of magnitude, resistance elements which have been produced in apowder metallurgical process, mainly from silicides, especiallymolybdenum silicide, possibly with a residue of oxides, carbides, suchas silicon carbide, and borides, in which process there may be used asthe metallic constituent, in addition to molybdenum, one or more of theelements Ti, Zr, V, Nb, Ta andv Cr (titanium, zirconium, vanadium,niobium (columbium), tantalum and chromium). When heating suchresistance elements to elevated temperatures, due to the action of theoxygen contained in the air, there will form on the same a superficiallayer of silicon dioxide which is slightly plastic and sticky atelevated temperatures so as to sinter together into a gastight filmwhich prevents further oxidation of the interior of the element.

Resistance elements made of such materials have fairly limitedmechanical strength characteristics which have, in the past, placed theuser before the alternative of either supporting the elements at aplurality of locations, or producing the same in lengths which arefrequently too short as compared to the furnace dimensions concerned.If, in accordance with the common practice in the field of metallicresistance elements, an attempt is made to support elements of the typeabove referred to directly upon the refractory liner of the furnaces,this will result in that the elements stick to the liner and will bepulled apart on thermal expansions occurring since they are not freelymovable. Such sticking, in addition to the action of the silicon dioxidefilm, may also be caused by the fact that the material of the elementshave often a tendency to react chemically with the base material whileforming reaction products having softening temperatures which are lowerthan the operating temperature of the elements. It should be noted inthis connection that this sticking is not at all tied up with the degreeof refractoriness of the liner material per se.

In order to avoid the drawback referred to, attempts have been made tomount the elements in cantileverfashion so that there would be no directcontact between the elements and the refractory material within theheating zones of the elements. However, this necessitates that highdemands must be placed on the mechanical strength characteristics of theelements which may lead to dimensions which are impractical from anelectrical point of view.

The present invention has for its object to overcome the drawbackreferred to and mainly resides in that the resistance element, composedsubstantially of silicidcs, preferably molybdenum silicide, possiblywith a residue of oxides, carbides and borides, is loosely suptheelement.

2,913,695 Patented Nov. 17, 1959 ported on a bed which is only inpartial contact with the element and which consists of relativelymovable grains of a refractory and electrically insulating materialwhich is chemically resistant to the material of the resistance element,also at the operating temperatures concerned. This arrangement providesfor a positive and satisfactory support for the elements, and thesupporting bed acts to prevent the same from sticking to the furnaceliner or refractory walls. The grains of the bed should be of arefractoriness suflicient to maintain the free mobility between thesingle grains up to a temperature of at least 1000" C., and in any caseup to the operating temperature concerned. It is preferable that thegrain quality should ensure the free mobility relative to each other upto a temperature of at least 1400 C. Owing to the fact that the materialof the bed is chemically resistant to the material of the elements, thelatter will be protected against chemical attacking at the points atwhich they contact the bed. Owing to the fact that the electricalresistance of the bed material is sufiiciently high, no leakage currentsin the bed will be capable of causing injury to the elements at theircontact points with the bed. Due to the remaining requirements which areplaced on the material of the bed as regards refractoriness andresistance to chemical attack, no difliculty will be encountered, as arule, in satisfying also the demand for a high electrical resistance.

In operation some of the grains in the portion of the bed immediatelyadjacent the element will stick to the silicon dioxide film of thelatter. When subsequently the length of the element changes due totemperature variations, the grains thus stuck to the element Will movetogether with the element and will thus slide over the readily movablegrains of the bed which do not stick to In this way the element will berelieved of any destructive mechanical strains.

In accordance with the invention, the bed material may suitably consistof one or more of the following substances, viz. silicon dioxide,silicon carbide or aluminum silicate. Particularly satisfactory resultshave been obtained using a bed of mullite or sillimanite. It is to beunderstood, however, that the invention is not restricted to theemployment of a bed material of this composition, since it is possibleto use any desired heatresistant material having a high electricresistance. However, the bed material must consist of substances which,even at the highest operating temperature concerned, remain chemicallyindifferent against the material of the resistance element so that noreaction products will form. The substances which, in the broadestaspect of the invention, may be comprised in the supporting bed for theresistance elements are: carbides, borides, silicides, silicates andoxides, either singly or in combination with each other.

The major part of the grains of the supporting bed should be sphericalin shape, or should have beaded edges and corners, at least, so as tofacilitate their relative mobility. The grains should be screened intodefin'ite granular fractions through different screens sized, forinstance, according to the Tyler standard screen scale, it beingpreferable that the grains of each bed should be of one single granularfraction ranging, for instance, between two adjacent mesh numbers, orshould be of two adjacent granular fractions, at least, for instance onefraction having grain sizes ranging from 6 to 8 mesh, and one adjacentfraction having grain sizes ranging from 4 to 6 mesh. It is understood,of course, that the average grain size should be so selected as toafford satisfactwice the average diameter of the grains contained in thebed. As a general statement, the grain sizes of the bed particles shouldrange between a minimum of 0.004" and a maximum of 0.4 (0.1 to 10 mm.).

It has been found that if the resistance elements are supported inaccordance with the teachings of the present invention, the elements canbe used also in conjunction with furnace liners which, when directlycontacted by the elements, would subject the elements to destructiveactions. Thus the present invention makes it possible to use refractoryliner materials of a standard quality selected exclusively taking intoaccount its refractoriness while disregarding any tendency of thematerial to react chemically with the element material at elevatedtemperatures.

Practical embodiments of the invention as applied to the manner ofmounting resistance elements in furnaces, are illustrated in theaccompanying drawings in which:

Fig. 1 is a vertical section through the bottom portion of a furnacewhereas Fig. 2 is a corresponding plan view shown partly broken away;

Figs. 3 and 4 are perspective scrap views showing a resistance elementdisposed on shelves formed on the internal wall surfaces of a furnace;

Fig. 5 is a side elevational view; and

Fig. 6 is a top end view of a supporting tube for a horizontallyextending resistance element, while Figs. 7 and 8 are cross-sectionstaken along the lines VII--VII and VHF-VIII, respectively, in Fig. 5;

Fig. 9 is a top plan view of the lower half of the refractory liner of atube-type furnace; and

Fig. 10 is a cross-section through the refractory liner, taken along theline X--X in Fig. 9.

In the embodiment illustrated in Figs. 1 and 2, the bottom 1 and sidewalls 2 to 5 of the furnace are made of a refractory material. Disposedon the bottom 1 is a bed 6 of a granular material consisting of aluminumsilicate of the approximative composition 65% A1 and 35% SiO Supportedhalf-way immersed into the bed 6 is a heating resistnace element 7consisting of 95% molybdenum silicide (Mosi the balance to 100%consisting of oxides and silicates. The resistance element is formed atits ends with enlarged terminal portions 8a, 8b passed through holesmade in the wall 2. The high-temperature heating zone proper of theelement 7 is formed by the smaller-diameter part thereof whereas itsterminal portions 8a, 3b will be at a considerably reduced temperaturedue to their larger cross-sectional area.

It would be possible, instead of using aluminum oxide and silicondioxide in the form of aluminum silicate as referred to above, to usepure granular silicon carbide of grain sizes ranging between 0.08 and0.2 inch (2 to mm.). It is suitable to use the so-called light greensilicon carbide since this is the purest form technically available. Asan alternative, a still further bed material may be employed, viz.powder metallurgically produced 0.16"

(4 mm.) pellets containing 15% by weight of very finely divided MoSi and82% by weight of silicon carbide.

As a further example of carrying-the invention into effect the case maybe mentioned, in which the resistance element 7 is composed of 90% byweight of MoSi 5% by weight of chromium boride (CrB), the balance beingI resistance element. Such a selection of material forfthe bed,therefore, would not be compatible with the condition stipulated thatthe bed material must be chemically resistant to the material of theresistance element.

In the embodiment illustrated in Figs. 3 and 4, the refractory wall 10'is disposed as a liner on the internal surface of a supporting brickwall 11 and is formed on its surface facing the furnace chamber withshelves 12, 13, etc. Disposed on these shelves are granular beds 14, 15,etc., and resting on the latter are two branches 7a, 7b, respectively,of the resistance element. These branches are interconnected by acantilevered intervening portion 7c. Fig. 3 illustrates the enlargedterminal portions 8c and 8d of the resistance element.

In the embodiment illustrated in Figs. 5 to 8 which is adapted formounting horizontally extending resistance elements, the resistanceelement 21, being bent into a hairpin-like configuration, rests on asupporting bed of a granular, electrically insulating material disposedin the tube 20, this material being refractory and chemically resistantto the material of the resistance element, also at the operatingtemperature concerned. The electric resistance of the bed 31 should besufficiently high to prevent the formation of any noticeable leakagecurrents. The material of the bed, as mentioned hereinbefore, has arefractoriness such as to allow the grains to conserve their relativemobility even at the highest temperatures concerned thereby providingfor flee thermal expansion and contraction lengthwise of the resistanceelement relative to the supporting tube.

The enlarged terminal portions 25 of the resistance element 21 arepassed out through, and fill out, holes made in a plug 26 of ceramicmaterial inserted in the corresponding end of the tube 20.

In Figs. 9 and 10 numeral 40 designates the upper and 41 the lower halfof the ceramic liner. Designated by 42 is a ceramic tube which isinserted in the furnace liner and defines the furnace chamber 43. Formedin the lower half 40 of the furnace liner are a plurality-six in numberin the embodiment illustrated-axially extending open channel-grooves 44each of which forms a shelf. Disposed on the bottom portion of each suchgroove or shelf 44 is a supporting bed 45 consisting of a granularelectrically insulating material which is refractory and chemicallyresistant to the material of the resistance element also at theoperating temperature concerned, as stated hereinbefore. Rested on therespective supporting beds 45 are the straight-lined portions 46 of alooped resistance element which is bent longitudinally at intervalsagainst a cylindrical surface. The uppermost two grooves or shelves 44are formed in the parting interface between the upper and lower halves40 and 41, respectively, of the, liner. The terminal portions 47 of theresistance elementthe cross-sectional area of which is larger than thatof the resistance conductor proper, 46, are introduced through holesmade in the rear wall 48 of the furnace.

Disposed in the front wall 49 of the furnace is the furnace entranceopening. The latter may be closed, in the conventional manner, by afurnace door which is not shown in the figure. The looped resistanceelement should be placed in the grooves 44 before the ceramic tube 42 ismoved longitudinally into its operative position. The resistance loopcan conveniently be removed for replacement or repair after firstwithdrawing the tube 42 from the furnace.

What is claimed is:

1. Electrical heating apparatus comprising an electrical resistanceheating element, and a bed of freely flowable refractory material grainshaving an electrical conductivity lower than that of the resistanceelement and chemically resistant to the material of said resistanceelement, said element resting uncovered upon the upper surface of saidbed in contact with said refractory material grains, and said bed beingoperative to support said element throughout substantially the fulllength thereof, and to simultaneously accommodate movement thereof dueto temperature-change induced expansion and contraction.

2. Apparatus as defined in claim 1 wherein the heating element consistsessentially of heat resistant and oxidation proof silicides.

3. Apparatus as defined in claim 1 wherein the heating element consistsessentially of molybdenum disilicide,

4. Apparatus as defined in claim 1 wherein the bed material consistsessentially of one or more of the materials in the group consisting ofheat-resistant carbides, borides, silicates, silicides and/ or oxides.

5. Apparatus as defined in claim 1 wherein the bed material comprisesone or more of the following substances: silicon dioxide, sillimaniteand mullite.

6. Apparatus as defined in claim 1 wherein the heating element consistsessentially of heat resistant and oxidation proof silicides, and the bedmaterial consists essentially of one or more of the materials in thegroup consisting of heat-resistant carbides, borides, silicates,silicides and/or oxides, said bed material including one or more of thefollowing substances: silicon dioxide, sillimanite and mullite.

7. Apparatus as defined in claim 1 wherein the heating element consistsessentially of molybdenum disilicide, and the bed material consistsessentially of sillimanite and mullite.

8. Apparatus as defined in claim 1 wherein the heating element isadapted for operating at temperatures above 1000 C., and the bedmaterial consists essentially of one or more of the materials in thegroup consisting of carbides, borides, silicates, silicides and/oroxides and having a sintering temperature higher than the operatingtemperature of the resistance element.

9. Apparatus as defined in claim 1 wherein the grain size of the grainsof the bed is comprised between 0.004 and 0.4 inch (0.1 to mm.).

10. Apparatus as defined in claim 1 wherein the bed is of a depth atleast equal to twice the average size of the grains of the bed.

11. Apparatus as defined in claim 1 wherein the References Cited in thefile of this patent UNITED STATES PATENTS 357,572 Burton Feb. 15, 1887941,339 Tone Nov. 23, 1909 1,019,568 Weintroub Mar. 5, 1912 1,025,144Kuhn May 7, 1912 1,044,295 Tone Nov. 12, 1912 1,158,972 Boeck Nov. 2,1915 1,597,900 Keene Aug. 31, 1926 1,645,867 Loughian Oct. 18, 19271,802,892 Hansen Apr. 28, 1931 1,878,619 Bechtel Sept. 20, 19321,894,685 Hediger Jan. 17, 1933 1,901,499 Fahrenwald Mar. 14, 19331,906,853 Hediger May 2, 1933 1,914,939 Boyer et al June 20, 19331,925,129 Boyles Sept. 5, 1933 2,001,297 Boyles May 14, 1935 2,235,091Vineberg Mar. 18, 1941 2,280,367 Barton Apr. 25, 1942 2,622,304 CofferDec. 23, 1952 2,678,958 Hintenberger May 18, 1954 2,744,946 Lewicki May8, 1956 FOREIGN PATENTS 648,331 France Aug. 3, 1928

